This invention relates to a controlled regenerative d-c power source which produces, from applied a-c power, d-c power for driving a load, the source being capable of returning, to the a-c power system, power which is generated in the load.
In a well-known prior regenerative d-c power supply, a phase-controlled SCR rectifier bridge, having a network of forward SCR's, rectifies applied a-c voltage from an a-c power system (either single-phase or three-phase) to develop a d-c voltage for energizing a load. Six forward SCR's are needed for three-phase, and four for single-phase. Normally, controlled power must be supplied to the load and this is accomplished by regulating the conduction angles of the forward SCR's during each half cycle of the applied a-c voltage. Each forward SCR can conduct, during each positive polarity half cycle of the voltage applied thereto from the a-c power system, when the SCR's anode is positive relative to its cathode. However, conduction will not occur during a positive half cycle until gate current is supplied to the SCR's gate. At that instant, the SCR fires into conduction, or turns on, and permits load current to flow therethrough until the end of the positive half cycle. The greater the phase angle or time delay between the start of a positive half cycle and the firing of the SCR into conduction, the less the conduction angle and the less alternating current that will be rectified and supplied to the load, thereby providing less d-c voltage across the output of the SCR rectifier bridge.
To achieve regeneration of power from the load and back into the a-c power system, each of the forward SCR's is shunted by a corresponding and oppositely poled reverse SCR, thereby requiring a total of twelve SCR's for three-phase and eight for single-phase. Reversing the power flow so that power will be translated from the load to the a-c power system is desirable in many applications where the load demand changes. For example, when the SCR rectifier bridge powers an inverter which in turn produces a-c voltage for driving, and controlling the speed of, an a-c induction motor, speed control is enhanced when it is possible to send power, generated in the load, back into the a-c power system. To explain, when the motor runs at a constant speed or the speed is increased, the load demand normally requires that power flow from the a-c power system to the inverter and motor. On the other hand, when the load requirements drop and it is necessary to decrease the motor speed in a relatively short time, such fast speed reduction cannot be accomplished unless the power flow can be reversed. This is necessary since the running motor acts as a generator, generating a counter electromotive force or counter EMF. Power must flow away from the motor to achieve fast speed reduction. In the prior regenerative d-c power supply, discussed above, when it is desired to reverse the power flow, the forward SCR's are turned off and the conduction angles of the reverse SCR's are controlled.
Unfortunately, a forward SCR and a series-connected similarly poled reverse SCR will always be across the two incoming power lines in the single-phase application and always across each pair of the three power lines in the three-phase environment. As a consequence, when a fault develops and those two SCR's are both in the conducting state at the same time, and when their anodes are positive relative to their cathodes, a line-to-line short circuit will be created and very high magnitude fault current will flow from the power system and through the SCR's. Of course, under normal conditions a forward SCR and a reverse SCR are never conductive simultaneously. However, an SCR may be inadvertently fired into conduction. For example, it can be triggered by noise. Since the forward and reverse SCR's can establish unwanted line-to-line short circuits, they must be capable of handling the resulting very high magnitude fault current without suffering permanent damage. Hence, the SCR's must be sized accordingly.
The controlled regenerative d-c power supply of the present invention constitutes a significant improvement over those previously developed, especially over the prior arrangement described hereinbefore, being considerably simpler and less expensive in construction and requiring several fewer components. Moreover, any fault currents are of much lower amplitude, thereby avoiding the oversizing of circuit element which is necessary in the prior arrangement.